Transversely-excited film bulk acoustic resonators with structures to reduce acoustic energy leakage

ABSTRACT

Acoustic resonators, acoustic filter devices and methods of making the same. An acoustic resonator device includes a piezoelectric plate having front and back surfaces, and an interdigital transducer (IDT) on the front surface including interleaved fingers. An overlapping distance of the interleaved fingers defines an aperture of the acoustic resonator device. The device further includes a fast region between the aperture and a busbar of the IDT. The piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region. The second frequency is higher than the first frequency.

RELATED APPLICATION INFORMATION

This patent claim priority to provisional patent application No. 63/118,688, filed Nov. 26, 2020 entitled STRUCTURES FOR SUPPRESSION OF ELECTRODE END EFFECTS. This patent claim priority to provisional patent application No. 63/118,689, filed Nov. 26, 2020 entitled STRUCTURES WITH DUMMY FINGERS FOR SUPPRESSION OF ELECTRODE END EFFECTS. This patent claim priority to provisional patent application No. 63/136,203, filed Jan. 11, 2021 entitled PISTON MODE XBAR ON Z-CUT LITHIUM NIOBATE. This patent claim priority to provisional patent application No. 63/136,204, filed Jan. 11, 2021 entitled PISTON MODE XBAR ON ROTATED Y-CUT LITHIUM NIOBATE. All of the above provisional patent applications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3^(rd) Generation Partnership Project). Radio access technology for 5^(th) generation (5G) mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5 GHz and 6 GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectional views, and a detail view of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is a schematic block diagram of a band-pass filter using acoustic resonators.

FIG. 3A is a schematic plan view of an XBAR with structures to reduce acoustic energy leakage.

FIG. 3B is a cross-sectional view of the XBAR of FIG. 3A.

FIG. 3C is an alternative cross-sectional view of the XBAR of FIG. 3A.

FIG. 4 is a graph of conductance as a function of frequency for an XBAR with structures to reduce acoustic energy leakage.

FIG. 5 is a graph of admittance as a function of frequency for an XBAR with structures to reduce acoustic energy leakage.

FIG. 6 is a schematic plan view of an XBAR with dummy electrodes and structures to reduce acoustic energy leakage.

FIG. 7 is a graph of conductance as a function of frequency for an XBAR with dummy electrodes and structures to reduce acoustic energy leakage.

FIG. 8 is a graph of admittance as a function of frequency for an XBAR with dummy electrodes and structures to reduce acoustic energy leakage.

FIG. 9 is a flow chart of a method for fabricating a filter with XBAR with structures to reduce acoustic energy leakage.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional views of an XBAR 100. XBAR-type resonators such as the XBAR 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate may be Z-cut, which is to say the Z axis is normal to the front and back surfaces 112, 114. The piezoelectric plate may be ZY-cut, rotated Y-cut, rotated Z-cut or rotated YX-cut. XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of a substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm 115 may be contiguous with the piezoelectric plate around at least 50% of the perimeter 145 of the cavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be attached to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 under the diaphragm 115. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The term “busbar” means a conductor from which the fingers of an IDT extend. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 that spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may be more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers are greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 130. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 130. Similarly, the thicknesses of the IDT fingers and the piezoelectric plate in the cross-sectional views are greatly exaggerated.

Referring now to the detailed schematic cross-sectional view (Detail C), a front-side dielectric layer 150 (or coating) may optionally be formed on the front side of the piezoelectric plate 110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer 150 may be formed only between the IDT fingers (e.g. IDT finger 138 b) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger 138 a). The front-side dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide, alumina, or silicon nitride. A thickness of the front side dielectric layer 150 is typically less than about one-third of the thickness tp of the piezoelectric plate 110. The front-side dielectric layer 150 may be formed of multiple layers of two or more materials. In some applications, a back-side dielectric layer (not shown) may be formed on the back side of the piezoelectric plate 110.

The IDT fingers 138 a, 138 b may be one or more layers of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum, chromium, titanium or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. Thin (relative to the total thickness of the conductors) layers of metals such as chromium or titanium may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars (132, 134 in FIG. 1) of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension m is the width or “mark” of the IDT fingers. The geometry of the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT may be 2 to 20 times the width m of the fingers. The pitch p is typically 3.3 to 5 times the width m of the fingers. In addition, the pitch p of the IDT may be 2 to 20 times the thickness of the piezoelectric plate 210. The pitch p of the IDT is typically 5 to 12.5 times the thickness of the piezoelectric plate 210. The width m of the IDT fingers in an XBAR is not constrained to be near one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be readily fabricated using optical lithography. The thickness of the IDT fingers may be from 100 nm to about equal to the width m. The thickness of the busbars (132, 134) of the IDT may be the same as, or greater than, the thickness of the IDT fingers.

FIG. 2 is a schematic circuit diagram and layout for a high frequency band-pass filter 200 using XBARs. The filter 200 has a conventional ladder filter architecture including three series resonators 210A, 210B, 210C and two shunt resonators 220A, 220B. The three series resonators 210A, 210B, and 210C are connected in series between a first port and a second port (hence the term “series resonator”). In FIG. 2, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 200 is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators 220A, 220B are connected from nodes between the series resonators to ground. A filter may contain additional reactive components, such as capacitors and/or inductors, not shown in FIG. 2. All the shunt resonators and series resonators are XBARs. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all of the series resonators are connected in series between an input and an output of the filter. All of the shunt resonators are typically connected between ground and one of the input, the output, or a node between two series resonators.

In the exemplary filter 200, the three series resonators 210A, 210B, and 210C and the two shunt resonators 220A and 210B of the filter 200 are formed on a single plate 230 of piezoelectric material bonded to a silicon substrate (not visible). In some filters, the series resonators and shunt resonators may be formed on different plates of piezoelectric material. Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 2, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 235). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity.

Each of the resonators 210A, 210B, 210C, 220A, and 220B in the filter 200 has resonance where the admittance of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter 200. In over-simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter's passband and the anti-resonance frequencies of the series resonators are positioned above the upper edge of the passband. In some filters, a front-side dielectric layer (also called a “frequency setting layer”), represented by the dot-dash rectangle 270, may be formed on the shunt resonators to set the resonance frequencies of the shunt resonators lower relative to the resonance frequencies of the series resonators. In other filters, the diaphragms of series resonators may be thinner than the diaphragms of shunt resonators. In some filters, the series resonators and the shunt resonators may be fabricated on separate chips having different piezoelectric plate thicknesses.

Three-dimensional simulations of XBAR devices show some acoustic energy may leak or be lost at the ends of the IDT fingers. A well-guided wave exhibits high order waveguide modes formed along the aperture. In a conventional structure, frequency of the acoustic wave in the gap region between the end of the busbar and alternating IDT fingers is lower because the wave has a larger even component since alternation in polarity is not induced well by a single electrode (i.e., finger). The transverse modes of the acoustic wave couple to the gap and can form energy confinement inside the gap. This appears either as a loss of strong spurs. Coupling to various transverse modes is possible because of the non-orthogonality between the uniform electric field along aperture and the transverse modes pattern. These transverse modes should be decoupled electrically by the IDT to suppress spurious modes.

Better resonator design is achieved by ensuring good waveguiding within a desired frequency band (e.g., between resonance and antiresonance). A “piston” design is where the fundamental transversal mode at resonance has uniform distribution along the aperture. Thus, the fundamental transverse mode pattern coincides with the external electric field pattern. In waveguide theory, waveguided modes are functionally orthogonal, thus all higher order modes are orthogonal to the fundamental mode pattern, which is the same as external electric field in piston mode. As a result, higher order transverse modes are electrically decoupled from the IDT.

FIG. 3A is a schematic plan view of an XBAR 300 with structures to reduce acoustic energy leakage. FIG. 3B is a cross-sectional view of the XBAR of FIG. 3A through Section D-D. Similar to the XBAR 100 of FIG. 1, XBAR 300 includes a piezoelectric plate 310 on a substrate 320 with a cavity 345, and an IDT 330 having interleaved fingers 336 extending from busbars 332, 334 on the piezoelectric plate 310. The interleaved fingers overlap for a distance AP, referred to as the “aperture” of the IDT.

The XBAR 300 also includes a fast region 352 proximate the busbars 332, 334. As shown in FIG. 3B, the piezoelectric plate 310 is thinner in the fast region 352 than the piezoelectric plate 310 in a central portion 356 of the aperture. A resonance frequency of the primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 in the fast region 352 has a higher frequency than the primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 within the central portion 356 of the aperture. In one example, the resonance frequency in the fast region 352 is greater than or equal to the anti-resonance frequency of the central portion 356. The frequency is higher in the thinner fast region 352 because frequency is approximately inversely proportional to diaphragm thickness. This results in reduced energy leakage from the ends of the fingers because improved waveguiding is achieved.

In one example, the piezoelectric plate has a thickness of 350 nm in the central portion, and the piezoelectric plate has a thickness of 310 nm in the fast region.

The XBAR 300 also includes a slow region 354 adjacent the fast region 352 and proximate an edge of the aperture. As shown in FIG. 3B, in the slow region 354, a strip 360 of dielectric material (e.g., SiO₂) is on the piezoelectric plate 310 such that the strip 360 is over a portion of the fingers 336 in the slow region 354. The primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 in the slow region 354 has a lower frequency than the primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 within the central portion 356 of the aperture. The frequency is lower in the thicker slow region 354 because frequency is approximately inversely proportional to diaphragm thickness. This results in reduced energy leakage from the ends of the fingers because improved waveguiding is achieved.

FIG. 3C is an alternative cross-sectional view of the XBAR of FIG. 3A through Section D-D. XBAR 300′ has a frontside dielectric layer 350 on the piezoelectric plate 310 over the IDT 330. The XBAR 300′ also includes a fast region 352′ proximate the busbars 332, 334. In the fast region 352′, the frontside dielectric layer 350 is thinner than the front-side dielectric layer 350 in a central portion 356′. The XBAR 300′ also includes a slow region 354′ adjacent the fast region 352′ and proximate an edge of the aperture. As shown in FIG. 3C, in the slow region 354′, an additional strip 360′ of dielectric material (e.g., SiO₂) is on the piezoelectric plate 310 such that the strip 360′ is over a portion of the fingers 336 in the slow region 354′.

Alternatively, a thickness of the piezoelectric plate, a thickness of the dielectric layer and/or a thickness of the interleaved fingers can be less in the fast region than in the central portion of the aperture to achieve the higher frequency. In another example, the mark of the IDT can be wider in the fast region than the central portion to achieve the higher frequency. Further, a thickness of the piezoelectric plate, a thickness of the dielectric layer, and a thickness of the interleaved fingers can be greater in the slow region than in the central portion of the aperture to achieve the lower frequency. In another example, the mark of the IDT can be narrower in the slow region than the central portion to achieve the lower frequency. To achieve desired performance of the device, the thicknesses of all of the layers (i.e., the piezoelectric plate, the dielectric layer, and the IDT fingers) of the device can be optimized in the fast region, slow region, and central portion according the resonance frequencies and the materials forming the layers. Further, the lengths (in a direction along the fingers) of the fast region and slow region can be optimized to achieve desired importance.

In other examples, the fast region can either extend under the busbars, start at an edge of the busbars, or be positioned within a gap between the busbars and the fast region. The fast region may or may not extend under the ends of the alternating fingers extending from the opposite busbar. The slow region can be directly adjacent to the fast region, or there may be a gap between the fast region and the slow region. The slow region can extend into a gap region between the ends of alternating fingers extending from the opposite busbar, can begin at the ends of these fingers, or can be entirely within the aperture.

In one particular example, the piezoelectric plate is 391 nm thick ZY-cut lithium niobite, the slow region has a length (in a direction along the fingers) of pitch/2 and a 40 nm thick strip of SiO₂ over the piezoelectric plate and the fingers, and the fast region has a length (in a direction along the fingers) of 6 microns where the piezoelectric plate has been thinned to 341 nm. In exemplary devices, the fast region has an effective thickness up to 20% less than the thickness of the central portion, where the effective thickness includes all layer (e.g., piezoelectric plate, dielectric layers, and any other layers). The fast region has a width of p to 3p in the aperture direction. In the fast region, the finger dimensions may be up to 35% wider and up to thinner 25% than the central portion. The slow region has an effective thickness up to 20% thicker than the central portion. The width of the slow region along the aperture direction can be between 0.25p and 2p. The central portion thickness is such that its resonance frequency matches the required frequency.

FIG. 4 is a graph 400 of simulated conductance as a function of frequency for XBARs with and without structures to reduce acoustic energy leakage. Curve 410 shows conductance for an XBAR with a uniform ZY-cut lithium niobate piezoelectric plate having thickness tp. Curve 420 shows conductance for an XBAR with a ZY-cut lithium niobate piezoelectric plate having a thickness tp in a central portion, a 3 micron long (in a finger direction) slow region having a piezoelectric plate thickness tp and a 0.1 tp thick strip of SiO₂ on the piezoelectric plate and fingers, and a 3 micron long (in a finger direction) fast region having a piezoelectric plate thickness 0.87 tp. Curve 420 shows reduced spurious modes and lower loss as compared to curve 410.

FIG. 5 is a graph 500 of admittance as a function of frequency for XBARs with and without structures to reduce acoustic energy leakage. Curve 510 shows admittance for an XBAR with a uniform ZY-cut lithium niobate piezoelectric plate having thickness tp. Curve 520 shows conductance for an XBAR with a ZY-cut lithium niobate piezoelectric plate having a thickness tp in a central portion, a 3 micron long (in a finger direction) slow region with having a piezoelectric plate thickness tp and a 0.1 tp thick strip of SiO₂ on the piezoelectric plate and fingers, and a 3 micron long (in a finger direction) fast region having a piezoelectric plate thickness 0.87 tp. Curve 520 shows reduced spurious modes and lower loss as compared to curve 510.

FIG. 6 is a schematic plan view of an XBAR 600 with dummy electrodes and structures to reduce acoustic energy leakage. Similar to FIGS. 6A, 6B, and 6C, XBAR 600 includes a piezoelectric plate 610, and an IDT 630 having interleaved fingers 636 extending from busbars 632, 634 on the piezoelectric plate 610. The interleaved fingers overlap for a distance AP, referred to as the “aperture” of the IDT. Further, dummy fingers 680 extend alternately from the busbars 632, 634 into a gap between the end of a finger 636 and the busbar 632, 634.

The XBAR 600 also includes a fast region 652 proximate the busbars 632, 634 where the piezoelectric plate 610 is thinner than the piezoelectric plate 610 in a central portion 656 of the aperture AP. The primary shear acoustic mode excited by the IDT 630 in the piezoelectric plate 610 in the fast region 652 has a higher frequency than the primary shear acoustic mode excited by the IDT 630 in the piezoelectric plate 610 within the central portion 656 of the aperture. This results in reduced energy leakage from the ends of the fingers because improved waveguiding is achieved.

The XBAR 600 also includes a slow region 654 adjacent the fast region 652 and proximate an edge of the aperture. The slow region 654 includes a strip (not shown) of dielectric material (e.g., SiO₂) on the piezoelectric plate 610 such that the strip 660 is over the portion of the fingers 636 in the slow region 654. The primary shear acoustic mode excited by the IDT 630 in the piezoelectric plate 610 in the slow region 654 has a lower frequency than the primary shear acoustic mode excited by the IDT 630 in the piezoelectric plate 610 within the central portion 656 of the aperture. This results in reduced energy leakage from the ends of the fingers because improved waveguiding is achieved.

The dummy fingers can be metal (e.g., the same or different metal as the fingers) and/or one or more other materials such as SiO_(s) or other dielectrics. The dummy fingers can have various shapes, such as a hammerhead shape with a thicker portion away from the busbar and a thinner portion near the busbar. The dummy fingers can have dimensions optimized to improve functioning of the device (e.g., to improve waveguiding). Exemplary dummy fingers typically have the mark and thickness of the finger in the central portion, but can vary by +/−30%. A length along the aperture direction is up to the width of the fast region, e.g., p to 3p. A space between a dummy finger and a tip of a finger may have a length from 300 nm to p/2.

FIG. 7 is a graph 700 of simulated conductance as a function of frequency for XBARs with dummy fingers with and without structures to reduce acoustic energy leakage. Curve 710 shows conductance for an XBAR with a uniform ZY-cut lithium niobate piezoelectric plate diaphragm having thickness tp. Curve 720 shows conductance for an XBAR with a ZY-cut lithium niobate piezoelectric plate diaphragm having a thickness tp in a central portion, a p/2 long (in a finger direction) slow region with having a piezoelectric plate thickness tp and a 0.06 tp thick strip of SiO₂ on the piezoelectric plate and fingers, and a 0.5 micron long (in a finger direction) fast region having a piezoelectric plate thickness 0.95 tp. Curve 720 shows reduced spurious modes and lower loss as compared to curve 710.

FIG. 8 is a graph 800 of admittance as a function of frequency for XBARs with dummy fingers with and without structures to reduce acoustic energy leakage. Curve 710 shows admittance for an XBAR with a uniform ZY-cut lithium niobate piezoelectric plate diaphragm having thickness tp. Curve 720 shows conductance for an XBAR with a ZY-cut lithium niobate piezoelectric plate diaphragm having a thickness tp in a central portion, a p/2 long (in a finger direction) slow region with having a thickness tp and a 0.06 tp thick strip of SiO₂ on the piezoelectric plate and fingers, and a 0.5 micron long (in a finger direction) fast region having a thickness 0.95 tp. Curve 720 shows reduced spurious modes and lower loss as compared to curve 710.

Description of Methods

FIG. 9 is a simplified flow chart summarizing a process 900 for fabricating a filter device incorporating XBARs with structures for reducing acoustic energy leakage. Specifically, the process 900 is for fabricating a filter device including multiple XBARs, some of which may include a frequency setting dielectric or coating layer. The process 900 starts at 905 with a device substrate and a thin plate of piezoelectric material disposed on a sacrificial substrate. The process 900 ends at 995 with a completed filter device. The flow chart of FIG. 9 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 9.

While FIG. 9 generally describes a process for fabricating a single filter device, multiple filter devices may be fabricated simultaneously on a common wafer (consisting of a piezoelectric plate bonded to a substrate). In this case, each step of the process 900 may be performed concurrently on all of the filter devices on the wafer.

The flow chart of FIG. 9 captures three variations of the process 900 for making an XBAR which differ in when and how cavities are formed in the device substrate. The cavities may be formed at steps 910A, 910B, or 910C. Only one of these steps is performed in each of the three variations of the process 900.

The piezoelectric plate may typically be ZY-cut or rotated YX-cut lithium niobate. The piezoelectric plate may be some other material and/or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process 900, one or more cavities are formed in the device substrate at 910A, before the piezoelectric plate is bonded to the substrate at 915. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 910A will not penetrate through the device substrate.

At 912, the piezoelectric plate is optionally thinned in a fast region. In some examples, a back surface of the piezoelectric plate is thinned to form the fast region. The thinning of the piezoelectric plate in the fast region can be achieved by any suitable means, such as etching, photolithographic, or mechanical techniques.

At 915, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the device substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or intermediate material layers.

At 920, the sacrificial substrate may be removed. For example, the piezoelectric plate and the sacrificial substrate may be a wafer of piezoelectric material that has been ion implanted to create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric plate and the sacrificial substrate. At 920, the wafer may be split along the defect plane, for example by thermal shock, detaching the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. The exposed surface of the piezoelectric plate may be polished or processed in some manner after the sacrificial substrate is detached.

A first conductor pattern, including IDTs and reflector elements of each XBAR, is formed at 945 by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. Optionally, the conductor layer may be thinner at the fast region and thicker at the slow region. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor pattern (for example the IDT bus bars and interconnections between the IDTs).

Each conductor pattern may be formed at 945 by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 945 using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

The conductor pattern can be formed such that the conductor pattern is thinner in the fast regions and/or thicker in the slow regions than the rest of the conductor pattern.

Optionally, dummy fingers may be formed in gap regions when the conductor pattern is formed. The dummy fingers may be formed of the same material as the conductor pattern. Alternatively, dummy fingers may be formed of different material than the conductor pattern, such as a dielectric material like SiO₂.

At 950, one or more frequency setting dielectric layer(s) may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. For example, a dielectric layer may be formed over the shunt resonators to lower the frequencies of the shunt resonators relative to the frequencies of the series resonators. The one or more dielectric layers may be deposited using a conventional deposition technique such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate. For example, a mask may be used to limit a dielectric layer to cover only the shunt resonators.

Also at 950, strips may be formed over the slow region when the dielectric layers are formed, where the strips are thicker layers of the dielectric material. The strips can be the same material or a different material from the dielectric material. Further, the dielectric layer over the fast region can be formed to be thinner than the rest of the dielectric layer.

At 955, a passivation/tuning dielectric layer is deposited over the piezoelectric plate and conductor patterns. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connections to circuitry external to the filter. In some instantiations of the process 900, the passivation/tuning dielectric layer may be formed after the cavities in the device substrate are etched at either 910B or 910C.

At 950 or 955, strips may be formed over the slow region when the dielectric layers are formed, where the strips are thicker layers of the dielectric material. The strips can be the same material or a different material from the dielectric material. Further, the dielectric layer over the fast region can be formed to be thinner than the rest of the dielectric layer.

In a second variation of the process 900, one or more cavities are formed in the back side of the device substrate at 910B. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the device substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in FIG. 1.

In a third variation of the process 900, one or more cavities in the form of recesses in the device substrate may be formed at 910C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities formed at 910C will not penetrate through the device substrate.

Ideally, after the cavities are formed at 910B or 910C, most or all of the filter devices on a wafer will meet a set of performance requirements. However, normal process tolerances will result in variations in parameters such as the thicknesses of dielectric layer formed at 950 and 955, variations in the thickness and line widths of conductors and IDT fingers formed at 945, and variations in the thickness of the piezoelectric plate. These variations contribute to deviations of the filter device performance from the set of performance requirements.

To improve the yield of filter devices meeting the performance requirements, frequency tuning may be performed by selectively adjusting the thickness of the passivation/tuning layer deposited over the resonators at 955. The frequency of a filter device passband can be lowered by adding material to the passivation/tuning layer, and the frequency of the filter device passband can be increased by removing material to the passivation/tuning layer. Typically, the process 900 is biased to produce filter devices with passbands that are initially lower than a required frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation/tuning layer.

At 960, a probe card or other means may be used to make electrical connections with the filter to allow radio frequency (RF) tests and measurements of filter characteristics such as input-output transfer function. Typically, RF measurements are made on all, or a large portion, of the filter devices fabricated simultaneously on a common piezoelectric plate and substrate.

At 965, global frequency tuning may be performed by removing material from the surface of the passivation/tuning layer using a selective material removal tool such as, for example, a scanning ion mill as previously described. “Global” tuning is performed with a spatial resolution equal to or larger than an individual filter device. The objective of global tuning is to move the passband of each filter device towards a desired frequency range. The test results from 960 may be processed to generate a global contour map indicating the amount of material to be removed as a function of two-dimensional position on the wafer. The material is then removed in accordance with the contour map using the selective material removal tool.

At 970, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at 965. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from 960 may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to restrict the size of the areas from which material is removed. For example, a first mask may be used to restrict tuning to only shunt resonators, and a second mask may be subsequently used to restrict tuning to only series resonators (or vice versa). This would allow independent tuning of the lower band edge (by tuning shunt resonators) and upper band edge (by tuning series resonators) of the filter devices.

After frequency tuning at 965 and/or 970, the filter device is completed at 975. Actions that may occur at 975 include forming bonding pads or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at 945); excising individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 995.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic resonator device comprising: a piezoelectric plate having front and back surfaces; an interdigital transducer (IDT) on the front surface comprising interleaved fingers, an overlapping distance of the interleaved fingers defining an aperture of the acoustic resonator device, the back surface attached to a surface of a substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate; and a fast region proximate a busbar of the IDT, wherein the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region, and wherein the second frequency is higher than the first frequency; and wherein a thickness of the piezoelectric plate and a thickness of the diaphragm are each less in the fast region than in the central portion of the aperture.
 2. The device of claim 1 further comprising a slow region between the fast region and the central portion, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a third frequency in the slow region, and wherein the third frequency is lower than the first frequency.
 3. The device of claim 1, wherein the IDT further comprises dummy fingers in the fast region.
 4. The device of claim 1, wherein the fast region is configured to reduce acoustic energy leakage as compared to an acoustic resonator device without a fast region.
 5. An acoustic resonator device comprising: a piezoelectric plate having front and back surfaces; an interdigital transducer (IDT) on the front surface comprising interleaved fingers, an overlapping distance of the interleaved fingers defining an aperture of the acoustic resonator device; a dielectric layer on the piezoelectric plate; and a fast region proximate a busbar of the IDT, wherein the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region, wherein the second frequency is higher than the first frequency, and wherein a thickness of the piezoelectric plate and a thickness of the dielectric layer is less in the fast region than in the central portion of the aperture.
 6. The device of claim 5 further comprising a slow region between the fast region and the central portion, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a third frequency in the slow region, wherein the third frequency is lower than the first frequency, and wherein one or more of a thickness of the piezoelectric plate, a thickness of the dielectric layer, and a thickness of the interleaved fingers is greater in the slow region than in the central portion of the aperture.
 7. A filter device comprising: a piezoelectric plate having front and back surfaces; a conductor pattern on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, each of the plurality of IDTs comprising interleaved fingers, an overlapping distance of the interleaved fingers defining an aperture of a respective resonator of the plurality of resonators, the back surface attached to a surface of a substrate except for portions of the piezoelectric plate forming diaphragms spanning cavities in the substrate, wherein at least one of the plurality of resonators comprises of a fast region proximate a busbar of a respective IDT, wherein the piezoelectric plate and the respective IDT are configured such that a radio frequency signal applied to the respective IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region, wherein the second frequency is higher than the first frequency; and wherein a thickness of the piezoelectric plate and a thickness of the diaphragms are each less in the fast region than in the central portion of the aperture.
 8. The device of claim 7, wherein the at least one resonator further comprises a slow region between the fast region and the central portion, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a third frequency in the slow region, and wherein the third frequency is lower than the first frequency.
 9. The device of claim 7, wherein the respective IDT further comprises dummy fingers in the fast region.
 10. The device of claim 7 wherein the fast region is configured to reduce acoustic energy leakage as compared to a resonator device without a fast region.
 11. A filter device comprising: a piezoelectric plate having front and back surfaces; a conductor pattern on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, each of the plurality of IDTs comprising interleaved fingers, an overlapping distance of the interleaved fingers defining an aperture of a respective resonator of the plurality of resonators; a dielectric layer on the piezoelectric plate, wherein at least one of the plurality of resonators comprises of a fast region proximate a busbar of a respective IDT, wherein the piezoelectric plate and the respective IDT are configured such that a radio frequency signal applied to the respective IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region, wherein the second frequency is higher than the first frequency, and wherein a thickness of the piezoelectric plate and a thickness of the dielectric layer is less in the fast region than in the central portion of the aperture.
 12. The device of claim 11, wherein the at least one resonator further comprises a slow region between the fast region and the central region, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a third frequency in the slow region, wherein the third frequency is lower than the first frequency, and wherein one or more of a thickness of the piezoelectric plate, a thickness of the dielectric layer, and a thickness of the interleaved fingers is greater in the slow region than in the central portion of the aperture.
 13. A method of fabricating an acoustic resonator device comprising: attaching a substrate to a back side of a piezoelectric plate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate; and forming an interdigital transducer (IDT) on a front side of the piezoelectric layer, the IDT comprising interleaved fingers, an overlapping distance of the interleaved fingers defining an aperture of the acoustic resonator device, forming a fast region proximate a busbar of the IDT, wherein the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a first frequency in the piezoelectric plate within a central portion of the aperture and a primary shear acoustic mode having a second frequency in the fast region, wherein the second frequency is higher than the first frequency; and wherein a thickness of the piezoelectric plate and a thickness of the diaphragm are each less in the fast region than in the central portion of the aperture.
 14. The method of claim 13, wherein a slow region is between the fast region and the central portion, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode having a third frequency in the slow region, and wherein the third frequency is lower than the first frequency.
 15. The device of claim 13 further comprising a dielectric layer on the piezoelectric plate, wherein a thickness of the dielectric layer is less in the fast region than in the central portion of the aperture. 